1. Field of the Invention
This invention relates to miniature receivers and transmitters operating in the HF, VHF, and UHF band and more particularly to microstrip directional couplers utilized in the receivers and transmitters. Description of the Prior Art
U.S. Pat. No. 5,159,298 titled "Microstrip Directional Coupler with Single Element Compensation" describes a coupler which employs closed form solutions for the compensating capacitance or inductance and introduces a new odd mode characteristic impedance necessary to realize high directivity in microstrip directional couplers. The single capacitively-compensated microstrip directional coupler includes four ports and two symmetrical inner conductors separated by a gap on a dielectric substrate having a relative dielectric constant. At the far end of the coupled section, there is a lumped capacitor implemented on the microstrip. Therefore, capacitance or inductance connects two quarter wavelength coupled lines of a microstrip directional coupler to realize high directivity or match.
Prior art miniature devices have significant problems. These problems relate to decreasing dimensions (increasing of the integration level), the requirement to improve a coupling flatness and the tuning of the coupling and the center frequency. It is an object of the present invention to reduce overall size of the coupler, to improve coupling flatness in the equivalent bandwidth and to improve the tuning of the coupling and center frequency.